1. Field of the Invention
The present invention relates generally to electrical filters and in particular to electrical notch filters.
2. Description of the Related Art
A notch filter is a band elimination or band stop filter, for attenuating or damping a part of a frequency band centered on a center frequency. Stated otherwise, the amplitude response of a notch filter is flat at all frequencies except for the stop band on either side of the center frequency. The drastic attenuation of the filter response around the targeted frequency is called the notch.
Notch filters have been used for years in the signal processing of signals in a reverberant or high clutter background. A biquad notch filter is a notch filter with a two pole and two zero filter topology, i.e. with a s-domain transfer function with 2 as the highest exponent both in the numerator or denominator. Such a filter is characterized by the 2 poles and 2 zeros of its transfer function. The zeroes are directly linked to the band elimination capacities of the biquad notch filter.
Commonly used biquad notch filters require a lot of operational amplifiers (OP-Amp), resulting in complex circuit architectures and high current consumptions. They can also display very large components ratios, with subsequent matching difficulties, i.e. pole positioning.
Several known biquad notch filters are shown in the book “Analog Filter Design” by Van Valkenburg. The 4-OP-Amp biquad circuit, presented on page 136, for example, presents a high current consumption and its zero placement is not independent of the pole placement, as they are equal in magnitude. The Bainter circuit, found in Van Valkenburg's, page 348, also shows a high current consumption with its 3 Op-Amps. The Boctor circuit, on page 350 of Van Valkenburg's, with its 8 elements, offers considerable latitude in the choice of element sizes, though it still requires careful matching between elements to control zero positioning. The ratio between elements can be very large. The derivation of the Friend circuit, in Van Valkenburg's page 358 is only applicable for when the zero's magnitude is greater than the pole's magnitude. It is characterized by a very large resistor ratio, and presents attenuation at low frequencies.